1. Field of the Invention
The present invention relates to a multi-piece solid golf ball having a solid core and a cover which has a two-layer structure of an inner layer cover and an outer layer cover and is formed in such a manner as to cover the solid core. In particular, the present invention relates to a multi-piece solid golf ball, in which a polyurethane based elastomer is used as a main component of a material forming the outer layer cover, and a surface hardness of the solid core, a hardness of each of the inner and outer layer covers, a thickness of each of the inner and outer layer covers, and a dimple VR value are optimized.
2. Prior Art
In recent years, various golf balls characterized by using a resin cover having, at its outermost layer, a relatively soft characteristic have been developed as golf balls for professional golfers and experienced amateur golfers. Such gold balls have been described, for example, in Japanese Patent Laid-open Nos. Hei 6-343718, Hei 7-194736, and Hei 7-24085.
The golf ball described in Japanese Patent Laid-open No. Hei 6-343718 includes an inner layer cover made from a high acid ionomer and an outer layer cover made from a relatively soft low modulus ionomer or a non-ionomer thermoplastic elastomer such as a polyurethane elastomer. This golf ball is intended to increase the flight distance with no sacrifice both in ball characteristic required for a golf game and in durability of the golf ball.
The golf ball described in Japanese Patent Laid-open No. Hei 7-194736 is intended to increase the flight distance and particularly to enhance the stability of iron-shot and the feeling upon impact. This golf ball uses a zinc ion neutralization type ionomer as a main resin component of each of an inner layer cover and an outer layer cover.
Japanese Patent Laid-open No. Hei 7-24085 describes a three-piece solid golf ball, in which an ionomer resin such as xe2x80x9cHimilan 1650xe2x80x9d (sold by Du Pont-Mitsui Polychemicals Co., Ltd.) or xe2x80x9cSurlyn 8120xe2x80x9d (sold by E. I du Pont de Nemours and Company), or a polyester based thermoplastic elastomer such as xe2x80x9cHytrel 4047xe2x80x9d (sold by Du Pont-Toray Co., Ltd.) is as a material for forming an outer layer cover. This golf ball is intended to improve the flight performance, feeling characteristic, and durability.
These proposed golf balls, however, has a problem that since the outer layer cover is made from a relatively soft cover material such as an ionomer resin or a thermoplastic elastomer, even through the feeling upon impact and flight performance can be improved, the scratch resistance is poor. For example, after repeated hitting of the golf ball, flaws are liable to occur in the surface of the golf ball due to contact with an edge portion or a head portion of an iron or the like.
Another problem of the above-described known golf balls is that when grass or the like is sandwiched between a club face and the golf ball upon iron-shot, that is, upon so-called flier, the controllability of the golf ball is insufficient, and further, cracking may occur after repeated hitting of the golf ball, that is, the durability against repeated hitting of the golf ball is not sufficiently better.
To meet the above-described demands, the present inventor has made studies on a multi-piece solid golf ball including a solid core, and a cover which has a two-layer structure of an inner layer cover and an outer layer cover and is formed in such a manner as to cover the solid core, wherein the cover has, in its surface, a number of dimples. As a result, the present inventor has found that it is possible to enhance not only the flight performance, feeling characteristic, and the ball controllability upon normal impact with a short iron such as a #9 iron or a sand wedge (that is, in the case where no foreign matter is sandwiched between a club face and the golf ball) but also the ball controllability upon flier (when grass or the like is sandwiched between a club face and the golf ball), and to improve the scratch resistance and the durability against repeated hitting of the golf ball, by using a polyurethane based elastomer as a main component of a material for forming the outer layer cover, setting a surface Shore D hardness of the solid core in a range of 50 to 60, a Shore D hardness of the inner layer cover in a range of 53 to 61, and a Shore D hardness of the outer layer cover in a range of 52 to 58, and optimizing the hardness and thickness of each layer and the dimples such that an average value of the surface Shore D hardness of the solid core, the Shore D hardness of the inner layer cover, and the Shore D hardness of the outer layer cover is in a range of 53 to 58; a value obtained by dividing a thickness of the outer layer cover by a thickness of the inner layer cover is in a range of 0.48 to 1.00, and a total of the thickness of the outer layer cover and the thickness of the inner layer cover is in a range of 1.5 mm to 3.5 mm; and a dimple volume occupying ratio VR value is in a range of 0.66% to 0.85%.
Accordingly, the present invention provides the following multi-piece sold golf balls.
(1) A multi-piece solid golf ball including: a solid core; and a cover which has a two-layer structure of an inner layer cover and an outer layer cover and is formed in such a manner as to cover the solid core, the cover having, in its surface, a number of dimples; wherein the outer layer cover is made from a material containing a polyurethane based elastomer as a main component; a surface Shore D hardness of the solid core is in a range of 50 to 60, a Shore D hardness of the inner layer cover is in a range of 53 to 61, and a Shore D hardness of the outer layer cover is in a range of 52 to 58; an average value of the surface Shore D hardness of the solid core, the Shore D hardness of the inner layer cover, and the Shore D hardness of the outer layer cover is in a range of 53 to 58; a value obtained by dividing a thickness of the outer layer cover by a thickness of the inner layer cover is in a range of 0.48 to 1.00, and a total of the thickness of the outer layer cover and the thickness of the inner layer cover is in a range of 1.5 mm to 3.5 mm; and a dimple volume occupying ratio VR value is in a range of 0.66% to 0.85%.
(2) A multi-piece solid golf ball according to claim 1, wherein the Shore D hardness of the inner layer cover is higher than the Shore D hardness of the outer layer cover.
(3) A multi-piece solid golf ball according to claim 1 or 2, wherein the inner layer cover is made from a material containing a thermoplastic resin as a main component.
(4) A multi-piece solid golf ball according to any one of claims 1 to 3, wherein an isocyanate mixture obtained by dispersing an isocyanate compound having two or more isocyanate groups as function groups in one molecule in a thermoplastic resin substantially not reacting with the isocyanate groups is mixed in the polyurethane based elastomer for forming the outer layer cover.